Optical disc track access apparatus and method for optical disc reproducer

ABSTRACT

The present invention relates to a track access apparatus and a method by which an optical pick-up unit quickly accesses a destination track of an optical disc by controlling a tracking actuator using a back electromotive force by detecting the vibration of an objective lens, the position variation, and the distortion of an actuator which occur when the optical pick-up unit is moved across the tracks of the optical disc at a high speed. In the present invention, there are provided a photodetector for detecting the amount of the beams of a side spot and a main beam spot based on two sub-beams and one main beam, a differential amplifier for differentiating the two signals of the sub-beams and outputting a tracking error signal, a first phase/gain compensation unit for compensating the phase and gain of the tracking error signal, a position detection unit for detecting the position of the objective lens from two signals of the main beam, a second phase/gain compensation unit for compensating the phase and gain of the lens position detection signal, a microcontroller for outputting a switch control signal, a switch for selecting an output of a first or second phase/gain compensation unit based on the switch control signal, a power amplifier for amplifying the output of the switch, and a tracking actuator for driving the objective lens in the track direction and the radial direction of the tracks.

The present application is a continuation of U.S. patent applicationSer. No. 09/271,457 filed on Mar. 18, 1999 now U.S. Pat. No. 6,791,915for which priority is claimed under 35 U.S.C. § 120; and the presentapplication claims priority of Patent Application No. 1998-9371 filed inRepublic of Korea on Mar. 18, 1998; Patent Application No. 1998-16674filed in Republic of Korea on May 9, 1998; and Patent Application No.1998-31642 filed in Republic of Korea on Jul. 31, 1998, under 35 U.S.C.§ 119. The entire contents of each of these applications are hereinfully incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical disc track access apparatusand method, and in particular to an optical disc track access apparatusand method which are capable of quickly accessing a destination trackposition on an optical disc by detecting a vibration, position variationand level variation of an objective lens and a characteristic value ofan actuator when an optical pick-up moves across the tracks of anoptical disc at a high speed and of controlling a tracking actuator bygenerating a back electromotive force corresponding to the detectedvalues.

2. Description of the Background Art

An optical disc reproducer is an apparatus capable of reading a digitaldata signal recorded on an optical disc and reproducing the thuslyrecorded data signal. The construction of the optical disc reproducerwill be explained with reference to the accompanying drawings.

FIG. 1 illustrates a conventional optical disc reproducer which includesa spindle motor 2 rotating an optical disc 1, a pick-up unit 3 reading adigital data signal recorded on the optical disc 1, a sled motor 4moving the pick-up unit 3 in the radial direction of the optical disc 1,a motor driving unit 6 driving the motors 2 and 4, a filtering andwaveform shaping unit (R/F) 5 receiving a signal detected by the pick-upunit 3 and outputting a filtered and waveform shaped signal, a traversecounter 13 receiving a signal detected by the pick-up unit 3 andcounting the number of tracks of the optical disc 1 traversed by theoptical pick-up unit 3, a microcontroller 11 receiving an externallyinputted instruction signal IN and an output signal of the traversecounter 13, detecting the rotation speed of the sled motor 4 and themovement of the pick-up unit 3 and controlling each circuit forming theoptical disc reproducer, a servo unit 7 receiving the filtered andwaveform shaped signal from the filtering and waveform shaping unit(R/F)5, a focus error signal FE and a tracking error signal TE outputted fromthe pick-up unit 3 in accordance with the control of the microcontroller11 and controlling the motor driving unit 6 and a digital signalprocessor 8, the digital signal processor 8 recovering the filtered andwaveform shaped signal into a digital signal and processing the thuslyrecovered signal into a compressed video signal, an MPEG unit 9 decodingthe compressed video signal in accordance with the control of themicrocontroller 11, and a memory 12 temporarily storing the compressedvideo signal.

The pick-up unit 3 includes a laser diode LD outputting a light beamhaving a predetermined wavelength for detecting the digital data signalrecorded on the optical disc 1, an objective lens and optical devicesfor processing the light beam outputted from the laser diode, and aphotodetector for receiving the light passed thereto through theobjective lens and the optical devices and changing the thusly receivedlight to an electrical signal.

The operation of the thusly constituted conventional optical discreproducer will now be explained.

When the optical disc reproducer is to reproduce the signals recorded onthe optical disc 1, the optical disc 1 is rotated, and the opticalpick-up unit 3 is moved for thereby detecting the signals recorded onthe optical disc 1 and outputting a high frequency detection signal tothe filtered and waveform shaped unit (R/F) 5. The filtering andwaveform shaping unit(R/F) 5 shapes the high frequency detection signal.The servo unit 7 which receives the wave-shaped signal detects asynchronous signal from the wave shaped signal, and the synchronoussignal is outputted to the digital signal processor 8. The digitalsignal processor 8 recovers the wave shaped signal from the filteringand waveform shaping unit 5 into an “original” digital signal, and theMPEG unit 9 which receives the recovered digital signal outputs asynchronous video signal, so that the optical disc reproducer reproducesthe signals recorded on the optical disc.

The operation wherein the optical disc reproducer searches the datarecorded on the optical disc will next be explained.

When a user inputs an instruction signal into the microcontroller for apredetermined data to be reproduced from a track at another positionbased on the current track of the optical disc, the microcontroller 11detects the position information, at which the pick-up unit 3 iscurrently positioned from the digital signal inputted from the digitalsignal processor 8 for thereby computing a track number of the opticaldisc corresponding to the position information detected.

At this time, the position information may be classified based on thekind of the disc. If the optical disc 1 is a compact disc(CD), theposition information may be a MSB(Minute, Second, block), and if theoptical disc 1 is a digital video disc(DVD), the position informationmay be a sector number.

After the microcontroller computes the track number of the destinationtrack at which the information recorded on the optical disc is to bereproduced, the difference between the current track number at which thepick-up unit is currently positioned, and the computed track number isobtained, and the moving direction is determined based on apredetermined result polarity.

The microcontroller 11 which determines the number of the tracks and themoving direction multiplies the number of the movement tracks by thewidth of a reference track for thereby computing the destination trackposition to which the pick-up unit 3 is to be moved and obtaining adistance over which the pick-up unit 3 is to be moved from the positionof the destination track. When the servo unit 7 controls the motordriving unit 6 so that the pick-up unit 3 is moved by a predetermineddistance in accordance with the control of the microcontroller, themotor driving unit 6 outputs a driving current to the sled motor 4 for apredetermined time in accordance with a control of the servo unit 7.Thereafter, the sled motor 4 is operated, and the pick-up unit 3 ismoved to the destination track position in the thusly determined movingdirection.

When the pick-up unit 3 is moved to the destination track position ofthe optical disc and reads the track information corresponding to themoved destination track position and outputs the thusly read trackinformation to the microcontroller 11, the microcontroller 11 judgeswhether the information requested by the user is the recordeddestination track. As a result of the judgement, if the user's requestedinformation is not the recorded destination track, a process whereby thetrack information is read from a new track position to which thepick-unit 6 is slightly moved is performed until the pick-up unit ismoved to the destination track position of the optical disc.

Actually, when the optical disc reproducer searches the informationrecorded on the optical disc, the sled motor is driven so that thepick-up unit is positioned over the destination track position for ashort time, and then the pick-up unit is moved across the tracks of theoptical disc. When the pick-up unit is moved across the tracks of theoptical disc, a vibration occurs in the optical devices of the pick-upunit, in particular, in the objective lens due to the inertia of thepick-up unit.

At this time, the vibration of the objective lens occurs because thephysical characteristic of the pick-up unit affects the actuator whichsupports the objective lens. In particular, the vibration is increasedwhen the pick-up is accelerated or decelerated. Since the vibrationwhich affects the objective lens directly affects the actuator whichcontrols the focus of the pick-up unit and the tracking operation, it isdifficult for the actuator to focus-control the pick-up unit. Inaddition, since the vibration of the objective lens affects the trackingcontrol after the pick-up unit is moved, much time is required theoptical disc reproducer to search the information recorded on theoptical disc.

In order to overcome the above-described problems, a conventionaltracking control circuit is employed. The construction and operation ofthe conventional tracking control circuit will next be explained.

FIG. 2 illustrates a first example of a conventional tracking controlcircuit which includes a photodiode 21 receiving two sub-beams from anoptical disc and converting a side spot light amount from the opticaldisc into electrical signals E and F, a differential amplifier 22obtaining the difference between the electrical signals E and F andgenerating a tracking error signal TE(=E−F), a phase/gain compensationunit 23 compensating the phase and gain of the tracking error signal TEand outputting a driving signal, a switch 24 switching the drivingsignal in accordance with a switching control signal SCS from themicrocontroller, a power amplifier 25 amplifying the driving signal fromthe switch 24, and a tracking actuator 26 receiving the amplifieddriving signal and adjusting the objective lens in the trackingdirection and radial tracking direction of the optical disc. Here, inthe tracking actuator 26, a certain distortion may occur due to thevibration of the objective lens.

In the above-described tracking control circuit, when the pick-up unit 3is moved across the tracks at a high speed, the switching operation ofthe switch 25 is controlled using the switching control signal SCSoutputted from the microcontroller, so that the tracking actuator 27 maynot be properly operated due to a noise component generated when theoptical pick-up unit moves across the tracks, thereby increasing thevibrations of the objective lens.

In order to overcome the above-described problems, as a second exampleof a conventional tracking control circuit disclosed in Japanese PatentPublication 7-311956(of Matsushita Electric Ind. Co. Ltd), a backelectromotive force corresponding to the destination track is applied tothe actuator in order to position the pick-up unit over the destinationtrack for thereby controlling the vibration of the objective lens.

As a third example of a tracking control circuit for overcoming theabove-described problems, Japanese Patent Publication 8-77572(ofMatsushita Electric Ind. Co. Ltd.) discloses a technique whereby thenumber of tracks and the moving direction are detected, and a backelectromotive force which is a predetermined off-set value correspondingto the detected moving direction and track number is applied to theactuator, so that the vibration of the objective lens of the pick-upunit is controlled.

However, in the above-described conventional tracking control circuitsof the optical disc reproducer, mechanical vibrations are generated dueto the inertial force generated by the wiring or actuator, and thethusly generated vibrations are increased when the optical pick-up unitis quickly accelerated or decelerated. Therefore, if the level of thevibration of the objective lens exceeds the capacity of the focuscontrol, the focus may become blurred. Even when the focus does notbecome blurred, it is impossible to fully control the trackingoperation, so that it takes a long time to search the informationrecorded on the optical disc.

In the conventional tracking control circuit of the optical discreproducer, when the optical pick-up unit is moved over the tracks at ahigh speed, and when controlling the vibrations of the objective lensdue to the inertia force which occurs when the optical pick-up unit isquickly accelerated or decelerated, the tracking control may exceed apredetermined limit, so that it is impossible to accurately control thefocus.

Therefore, in the conventional tracking control circuit, when theobjective lens is vibrated due to the inertial force, a predeterminedtime delay occurs from the time after the optical pick-up unit is moveduntil the time before the tracking servo is again under full control, sothat the time required for accessing the destination track of theoptical disc is increased.

In the optical disc reproducer disclosed in Japanese Patent Publications7-311956 and 8-77572, when the optical pick-up unit is moved over thetracks of the optical disc in order to access a destination track of theoptical disc at high speed, a maximum kick pulse is applied irrespectiveof the distance over which the optical pick-up unit is moved, and a backelectromotive force which varies the kick pulses in accordance with thedestination track delays the time for accessing the destination track.

In addition, in the conventional optical disc reproducer, a servo loopis needed for implementing a servo control by detecting the level of thevibration, thereby increasing the fabrication cost of the system.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide anoptical disc track access apparatus and a method of the same whichovercome the aforementioned problems encountered in the background art.

It is another object of the present invention to provide an optical disctrack access apparatus for an optical disc reproducer by which anoptical pick-up unit can quickly access a destination track of anoptical disc by accurately detecting a signal corresponding to the levelof a horizontal vibration with respect to the optical axis of anobjective lens when the objective lens is in a parallel state when anoptical pick-up unit of the optical disc reproducer is moved across thetracks of an optical disc, by adjusting the phase and gain of thedetected signal and applying a back electromotive force to an actuatorof the disc reproducer for compensating the level of the vibrations.

It is another object of the present invention to provide an optical disctrack access method for an optical disc reproducer by which an opticalpick-up unit can quickly access a destination track of an optical discby accurately detecting a signal corresponding to the level of ahorizontal vibration with respect to the objective lens axis when theobjective lens is in a parallel state when an optical pick-up unit ofthe optical disc reproducer is moved across the tracks of an opticaldisc, by adjusting the phase and gain of the detected signal andapplying a back electromotive force to an actuator of the optical discreproducer for compensating the level of the vibrations.

It is another object of the present invention to provide a method bywhich an optical pick-up unit of an optical disc reproducer can quicklyaccess the destination track of an optical disc on which an informationis recorded by accurately detecting a signal corresponding to thehorizontal vibration level with respect to the objective lens axis whenan objective lens is in a parallel state, by adjusting the phase andgain of the detection signal and applying a back electromotive forcecompensation control signal to an actuator of the optical discreproducer corresponding to the slope of the detection signal.

It is another object of the present invention to provide a method bywhich an optical pick-up unit of an optical disc reproducer can quicklyaccess the destination track of the optical disc on which an informationis recorded by accurately detecting a signal corresponding to the levelof the vibrations with respect to an objective lens axis when anobjective lens is in a parallel state, adjusting the phase and gain ofthe detection signal and applying a back electromotive signal to anactuator of the optical disc reproducer corresponding to the level ofthe detection signal.

It is another object of the present invention to provide a method bywhich an optical pick-up unit of an optical disc reproducer can quicklyaccess a destination track of the optical disc on which an informationis recorded by determining a characteristic value of an actuator of anoptical disc reproducer in accordance with a moving speed of a sledmotor of the optical disc reproducer and applying a back electromotiveforce to the actuator corresponding to the force applied to the actuatorand a distortion of the actuator based on the thusly determinedcharacteristic value.

In order to achieve the above objects, there is a provided an opticaldisc track access apparatus for an optical disc reproducer according toa first embodiment of the present invention which includes an opticalpick-up moving unit for moving an optical pick-up unit of the opticaldisc reproducer in a radial direction of a track of an optical disc, adetection unit for detecting a vibration of an objective lens of theoptical pick-up unit based on a reflected main light beam detected bythe optical pick-up unit, and a control unit for outputting acompensation signal to the optical pickup moving unit for decreasing thevibration of the objective lens based on an output signal from thedetection unit and thereby controlling the vibration of the objectivelens.

In order to achieve the above objects, there is also a provided anoptical disc track access apparatus for an optical disc reproduceraccording to a second embodiment of the present invention which includesan electrical signal generation unit for generating an electrical signalof a resonant frequency range detected by an optical pick-up unit of theoptical disc reproducer which moves in the radial direction of thetracks of an optical disc and having a resonant frequency range of atracking actuator of the optical disc reproducer, and a control unit forcontrolling the driving operation of the tracking actuator in accordancewith the thusly generated electrical signals.

In order to achieve the above objects, there is additionally a providedan optical disc track access apparatus for an optical disc reproduceraccording to a third embodiment of the present invention which includesa first tracking control unit for outputting a first tracking controlsignal based on a first auxiliary signal generated during the trackdirection movement of an optical pick-up unit of the optical discreproducer, a second tracking control unit for outputting a secondtracking control signal in accordance with a second auxiliary signal ofa frequency lower than the frequency of the first auxiliary signal andgenerated based on the movement of the optical disc in the radialdirection of the tracks of the optical pick-up unit, and a switchingunit for selecting one among the first and second tracking controlsignals and outputting the thusly selected signal in accordance with atrack search request.

In order to achieve the above objects, there is further a provided anoptical disc track access apparatus for an optical disc reproduceraccording to a fourth embodiment of the present invention which includesan auxiliary signal detection unit for detecting an auxiliary signal ofa resonant frequency range of a tracking actuator of the optical discreproducer based on the detection beam amount a plurality of beam spotregions reflected from an optical disc, an error signal generation unitfor generating a tracking error signal in accordance therewith, a backelectromotive force generation unit for generating a back electromotiveforce corresponding to the thusly generated tracking error signal, and adriving control unit for controlling a driving signal of the trackingactuator in accordance with the thusly generated electromotive force.

In order to achieve the above objects, there is still further a providedan optical disc track access method for an optical disc reproduceraccording to a fifth embodiment of the present invention which includesa detection step for detecting an auxiliary signal of a resonantfrequency range of a tracking actuator of the optical disc reproducerdetected by the optical pick-up unit of the optical disc reproducerwhich moves in a radial direction of the tracks of an optical disc, anda control step for controlling the driving operation of the trackingactuator in accordance with the detected auxiliary signal.

In order to achieve the above objects, there is also a provided anoptical disc track access method for an optical disc reproduceraccording to a sixth embodiment of the present invention which includesa first step for outputting a tracking control signal based on a firstauxiliary signal generated when an optical pick-up unit of the opticaldisc reproducer is moved in the track direction of an optical disc, asecond step for outputting a tracking control signal in accordance witha second auxiliary signal of a frequency lower than the frequency of afirst auxiliary signal and generated when the optical pick-up unit ismoved in a radial direction of the tracks of the optical disc, and athird step for selecting one signal among a plurality of trackingcontrol signals in accordance with a track search request and outputtingthe thusly selected signal.

In order to achieve the above objects, there is also a provided anoptical disc track access control method for an optical disc reproduceraccording to a seven embodiment of the present invention which includesa setting step for setting a tracking reference value corresponding to afirst auxiliary signal generated when an optical pick-up unit is movedin the track direction of an optical disc, a comparison step fordetecting a second auxiliary signal generated when the optical pick-upunit of the optical disc reproducer is moved in the radial direction ofthe tracks of the optical disc and comparing the thusly detected secondauxiliary signal with a tracking reference signal, and an output stepfor outputting a tracking control signal corresponding to a result ofthe comparison.

In order to achieve the above objects, there is further a provided anoptical disc track access method for an optical disc reproduceraccording to a eighth embodiment of the present invention which includesa detection step for detecting a second auxiliary signal generated whenan optical pick-up unit of the optical disc reproducer is moved in theradial direction of the tracks of an optical disc when an mode ischanged to a fine search mode, and an output step for outputting atracking control signal by which a number of the jump tracks per movingstep of the optical pick-up unit is varied in accordance with a resultof the detection.

In order to achieve the above objects, there is also a provided anoptical disc track access method for an optical disc reproduceraccording to an ninth embodiment of the present invention which includesdetermining a current acceleration in accordance with the variation ofthe current moving speed of an actuator of the optical disc reproducer,determining a degree of the distortion of the actuator corresponding tothe current acceleration based on a previously determined characteristicvalue of the actuator, and controlling the driving operation of theactuator in accordance with the thusly determined degree of thedistortion.

Additional advantages, objects and features of the invention will becomemore apparent from the description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

FIG. 1 is a schematic block diagram illustrating the components of aconventional optical disc reproducer;

FIG. 2 is a schematic block diagram illustrating the components of aconventional tracking control circuit;

FIG. 3 is a schematic block diagram illustrating the components of atrack access control apparatus according to a first embodiment of thepresent invention;

FIGS. 4A and 4B are plan and side views, respectively, illustrating atracking actuator for driving an objective lens;

FIGS. 5A through 5C are respective views illustrating a trajectory of amain beam inputted into a photodetector in accordance with a positionvariation of an optical device of an optical pick-up unit;

FIG. 6 is a view illustrating three light spots on the optical disc,which are generated by an optical pick-up unit using a 3-beam method;

FIG. 7 is a waveform diagram illustrating an output signal of a positiondetection unit;

FIG. 8A is a schematic circuit diagram illustrating a position detectionunit;

FIG. 8B is a schematic circuit diagram illustrating a second phase/gaincompensation unit;

FIG. 9 is a flow chart illustrating a track access control method of anoptical disc reproducer for compensating a vibration of an objectivelens according to a second embodiment of the present invention;

FIG. 10 is a flow chart illustrating a track access control method of anoptical disc reproducer according to a third embodiment of the presentinvention;

FIG. 11A is a flow chart illustrating a first tracking controlstep(coarse search) in the method of FIG. 10;

FIG. 11B is a flow chart illustrating a second tracking controlstep(fine search) in the method of FIG. 10;

FIG. 12A illustrates the output signal waveforms of the photodetectorfor detecting the position of the objective lens when the opticalpick-up unit of the optical disc reproducer moves in the radialdirection of the tracks of the optical disc in the stable state, andFIG. 12B is a wave form diagram illustrating an output signal of aposition detection unit which detects the position of the objectivelens;

FIG. 13 is a flow chart illustrating a track access control method of anoptical disc reproducer for controlling a tracking operation using atime-based variation of an objective lens in accordance with a fourthembodiment of the present invention;

FIG. 14A illustrates the output signal waveforms of the photodetectorfor detecting the position of the objective lens when the opticalpick-up unit of the optical disc reproducer moves in the radialdirection of the tracks of the optical disc in a stable state, and FIG.14B is a wave form diagram illustrating an output signal of a positiondetection unit which detects the position of the objective lens;

FIG. 15 is a flow chart illustrating a track access control method of anoptical disc reproducer for controlling a tracking operation using avibration level variation of an objective lens with respect to timeaccording to a fifth embodiment of the present invention;

FIG. 16 is a flow chart illustrating a track access control method of anoptical disc reproducer in the fine search mode in accordance with asixth embodiment of the present invention;

FIG. 17A is a graph illustrating the speed at which an optical pick-upunit is moved in a radial direction of the tracks, of an optical disc,and FIG. 17B is a graph illustrating an acceleration when an opticalpick-up unit is moved in a radial direction of the tracks;

FIG. 18 is a block diagram for explaining the computation of an actuatorcharacteristic value(K); and

FIG. 19 is a flow chart illustrating a tracking control method of anoptical disc reproducer utilizing the distortion of an actuator inaccordance with a seventh embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 3, the track access control apparatus according to afirst embodiment of the present invention includes a photodetector 30detecting two sub-beams E and F and one main beam reflected from sidespots and a main spot of an optical disc, respectively, and outputtingcorresponding electrical signals (E, F, A, B, C, D: since the one mainbeam is made incident onto a segmented photodetector), a differentialamplifier 31 outputting tracking error signal TE (=E−F) from theelectrical signals E and F from the photodetector 30, a first phase/gaincompensator unit 32 compensating the phase and gain of the trackingerror signal TE and outputting a first driving signal, a positiondetection unit 33 receiving the electrical signals A and C from thephotodetector 30, detecting therefrom the position of the objective lensand outputting a vibration level signal of the objective lens, a secondphase/gain compensation circuit 34 compensating the phase and gain ofthe vibration level signal of the objective lens from the positiondetection unit 33 and outputting a second driving signal, a switch 35selecting and outputting the first driving signal or the second drivingsignal in accordance with a switching control signal SCS from amicrocontroller (not shown), a power amplifier 36 amplifying the firstdriving signal or the second driving signal from the switch 35, and atracking actuator 37 adjusting the objective lens in the trackingdirection and radial direction of the tracks of the optical disc inaccordance with the amplified first driving signal or the amplifiedsecond driving signal. In the drawings, “r” indicated at the actuator 37represents the distortion (or vibration width) of the actuator 37.

FIGS. 4A and 4B illustrate the tracking actuator for driving theobjective lens which includes an objective lens 42 mounted with a bobbin44 on which is wound a coil 40 receiving a current applied thereto foradjusting the position of the objective lens 42 in accordance with theinputted current, and a magnetic member 43 with a magnet 41 forming amagnetic field.

FIGS. 5A through 5C illustrate the trajectories of the main beaminputted into the photodetector in accordance with a position variationof the optical devices of the optical pick-up unit. In each upperdrawing of FIGS. 5A through 5C, there are shown the objective lens 42, abobbin 44 fixing the objective lens 42, a support frame 52, and wires 51connecting the support frame and the bobbin 44. In each lower drawing,there are shown a photodetector 53 split onto four segments 54 as A, B,C and D onto which the main beam reflected by the optical disc andpassed through the objective lens is inputted.

In the segmented photodetector 53, when the optical pick-up unit ismoved toward the destination track position of the optical disc, thespot trajectory of the main beam is inputted along the axis of theobjective lens, and it is judged whether the optical pick-up unit ispositioned over the destination track position or is offset towards theinside or outside of the optical disc about the destination trackposition. If the objective lens 42 is not offset towards the innerdirection of the optical disc with respect to the beams inputted ontothe photodetector segments A and C, the position is determined based onthe degree of offset towards the outer direction. If there is adifference in the detected intensities of the beams, it is possibletherefrom to detect the position of the objective lens. Theabove-mentioned detection technique will now be explained.

FIG. 5B illustrates a trajectory wherein the spot trajectory of the mainbeam reflected by the optical disc and then passed through the objectivelens is formed on the center of the photodetector 53. In thephotodetector 53, since the beam amount incident on the segment A andthe beam amount incident on the segment C are the same, it is possibleto detect that the objective lens 42 is not offset toward the inner orouter directions of the optical disc.

FIG. 5A illustrates that the spot trajectory of the main beam reflectedby the optical disc and passed through the objective lens is offsettowards segment C of the photodetector 53. In the photodetector 53,since the beam amount incident on the segment A is smaller than the beamamount incident on the segment C, it is possible to detect that theobjective lens 42 is offset towards the inner direction of the opticaldisc. As shown in FIG. 5C, the photodetector 53 detects the caseopposite to that shown in FIG. 5A.

FIG. 6 illustrates the light beam spots that the optical pick-up unitforms on a optical disc using 3-beam method.

In the optical pick-up technique of the optical disc reproducer using a3-beam method, the data signal recorded on the optical disc isreproduced using one main beam positioned at the center portion, and twosub-beams E and F respectively formed towards the inner and outerportions of the main beam and generating a track error signal.

In the optical pick-up unit of an optical disc reproducer using the3-beam method, when the optical pick-up unit is moved from the innerportion to the outer portion of the data track of the optical disc, thenthe spots of the three beams are moved from the positions indicated bysolid lines to the positions indicated by the dashed lines, so that thespot of the main beam is positioned on the track for thereby reading thedata signal therefrom. In addition, the sub-beams E and F are slightlypositioned on the land in which there are pits. In this land, thesub-beams reflected by the land portion are converted into theelectrical signals by the photodetector and then these signals areinputted into the differential amplifier. At this time, at the dashedline position of the sub-beams, since the same amount of the beams isdetected, the track error signal is 0. Here, the sub-beams E and F areformed separately for the reason that the detection signal can beaccurately detected for thereby preventing crosstalk between theneighboring tracks.

FIG. 7 illustrates the waveform of an output signal of thephotodetector. When the optical pick-up unit moves across the tracks ofthe optical disc, the magnitude and polarity of the output signal of thephotodetector are changed in accordance with the variation of the beamspot. For example, if the output of the photodetector is a positivevalue(+), it means that the objective lens is offset towards the innerdirection, and if the output of the photodetector is a negativevalue(−), it means that the objective lens is offset towards the outerdirection. In view of the vibration of the objective lens, if theobjective lens is offset, the distortion degree and distortion distanceof the actuator will differ based on the offset distance.

When enlarging the waveform of the output signal of the photodetector,as indicated within the dashed circle, a low level signal component isrevealed. This low level signal is a noise component which is generatedwhen the position of the main beam is changed due to the vibration ofthe objective lens when the optical pick-up unit is moved across thetracks at a high speed.

The reasons for the generation of the noise component will be explainedin more detail.

The vibration of the objective lens occurs in relation with theoscillation period of the tracking actuator of the optical pick-up unit.The thusly generated vibrations are in the oscillation frequency rangeof the tracking actuator engaged with the objective lens. Generally,since the vibration of the optical pick-up unit has a frequencycharacteristic of tens of Hz, for example, since the vibration of the3-beam optical pick-up unit includes the above-described characteristic,and the error signal which is determined by whether the main beam ispositioned on the tracks is generated in proportion to the rotationspeed of the sled motor when the optical pick-up unit is moved acrossthe tracks at a high speed, frequencies of at least tens of kHz aregenerated. Therefore, the vibrations of the objective lens may changethe position of the main beam for thereby generating the noisecomponents. The thusly generated noise components are filtered.

FIG. 8(A) illustrates an exemplary circuit of the position detectionunit 33, which is formed of a differential amplification circuit forreceiving electrical signals A and C and outputting a position detectionsignal ag(C−A).

The differential amplification circuit comprising the position detectionunit includes an operational amplifier OP1 which receives the signals Aand C at its respective inverting and non-inverting input terminals viarespective input series resistors R1 and R2. A capacitor C1 in parallelwith a resistor R3 is connected between the non-inverting input terminaland ground for bypassing high frequency noise components riding on themain beam signal. Similarly, a negative feedback circuit composed of acapacitor C2 in parallel with a resistor R4 is connected between theoutput terminal and inverting input terminal of operational amplifierOP1 for applying negative feedback based on the position detectionsignal ag(C−A) outputted from the output terminal of operationalamplifier OP1.

FIG. 8B illustrates an exemplary circuit of the second phase/gaincompensation unit 34 which is formed of a integrator which integratesthe signal ag(C−A) from the position detection unit 33 over time, whichnegatively feeds back so that the tracking actuator operates in thereverse direction of the optical lens offset and compensates the phasegain of the control loop so as not to be oscillated under the operation.The integrator circuit includes an operational amplifier )OP2 having itsnon-inverting input terminal connected to ground. The position detectionsignal ag(C−A) is applied to the inverting input terminal of operationalamplifier OP2 via an input filter composed of a capacitor C11 and aresistor R11 connected in parallel, for blocking noise in the positiondetection signal ag. A buffer resistor R12 is connected in seriesbetween the inverting input terminal of operational amplifier OP2 andone terminal of a capacitor C12 the other terminal of which is connectedto the output terminal of operational amplifier OP2 at which thephase/gain compensation driving signal is outputted.

FIG. 9 illustrates the steps for implementing a tracking access methodfor compensating the vibration of the objective lens according to asecond embodiment of the present invention. In this method in Step S11,it is judged whether the pick-up unit is moved. If it is judged that thepick-up unit is moved, the beams A and C inputted into the photodetectorare detected in Step S12. The objective lens vibration signal isextracted from the electrical signals corresponding to a different beamamount in Step S13. Thereafter in Step S14, the driving signal isgenerated by compensating the phase and gain of the vibration levelsignal of the objective lens for adjusting the position of the objectivelens. Next, in Step S15, it is judged whether the switching controlsignal SCS is inputted from the microcontroller. If it is judged thatthe switching control signal SCS is inputted, the driving signal isoutputted by switching the switch in Step S16. Next, the driving signalis amplified in Step S17, and the driving signal is outputted to thetracking actuator in Step S18 for thereby controlling the position ofthe objective lens.

FIG. 10 illustrates the track access method for an optical discaccording to a third embodiment of the present invention. In Step S21,it is judged whether the optical pick-up unit is moved across the tracksfor searching the data recorded on the optical disc. If it is judgedthat the optical pick-up unit is moved, the Y-input terminal of theswitch is connected with the Z-output terminal in Step S22 for therebyimplementing a first tracking control for controlling the vibration ofthe objective lens in Step S23.

Next, if it is judged in Step S24 that the pick-up unit is not moved,the X-input terminal of the switch is connected with the Z-outputterminal in Step S25 for thereby performing a second trackingcontrol(fine search) in Step S26. The first and second tracking controlmethodologies will be explained with reference to FIG. 11.

FIG. 11A is a flowchart illustrating the sub-steps for implementing thefirst tracking control method(coarse search) of the method of FIG. 10.In Step S31, if the amount of the main beam reflected by the opticaldisc is detected, and then the electrical signals are outputted, thevibration amount signal of the objective lens is generated based on theelectrical signals in Step S32. A driving signal is generated byreceiving the vibration level signal and compensating the phase and gainfor generating a back electromotive force in Step S33, and then thedriving signal is amplified in Step S34. The thusly amplified drivingsignal is applied to the actuator for thereby controlling the trackingof the optical pick-up unit in Step S35.

FIG. 11B illustrates the sub-steps for implementing a second trackingcontrol(fine search) in the method of FIG. 10. In Step S41, when thephotodetector which detects the amount of the beam based on twosub-beams outputs an electrical signal, the electrical signals areinputted, and the tracking error signals E–F are generated in Step S42,and the phase and gain of the error signal are compensated in Step S43,and the compensated error signal is amplified for thereby generating adriving signal in Step S44, and then the amplified driving signal isapplied to the actuator for thereby controlling the tracking of theoptical pick-up unit in Step S45.

The method for controlling the tracking using the output signal from theposition detection unit of the objective lens, the track access methodfor controlling the tracking using the time-based variation(slope) ofthe objective lens position, and the track access method for controllingthe tracking using the vibration level variation of the objective lenswill be explained as follows.

FIG. 12A illustrates the output signal waveforms of the photodetectorfor detecting the position of the objective lens when the opticalpick-up unit of the optical disc reproducer moves in the radialdirection of the tracks of the optical disc in the stable state.

When the optical pick-up unit moves in the radial direction of thetracks in a normal(stable) state, the photodetector of the opticalpick-up unit due to the vibration of the actuator having an inherentresonant frequency (1/T1), outputs the detected signals CED1 and CED2and so on, which each has a different magnitude according to the movingspeeds of the optical pick-up unit of the optical disc reproducer.Therefore, the microcontroller calculates an average value CER of thedetected signals CED1 and CED2, as a reference value.

FIG. 12B illustrates the output signal waveforms of the positiondetection unit for detecting the position of the objective lens in whicheach slope respectively is indicated by SD1, SD11, SD2, SD21, SD3, SD31,SD4 and SD41 corresponding to their respective positions at the timest1, t2, t3 and t4 in the reference signal CER and the detected signalCED, respectively. At this time, if the slope of the output signal ofthe photodetector is greater than a reference slope in the positive andnegative values, the tracking of the objective lens is controlled byapplying the back electromotive force. The above-described controlmethod will be explained in detail with reference to FIG. 13.

FIG. 13 illustrates the steps for implementing a track access method forcontrolling the tracking using a time-based variation (slope) of theobjective lens position. When the optical pick-up unit runs at a normalstate (stable state), the optical pick-up unit is moved in the radialdirection of the tracks for thereby detecting the first auxiliary signalag(C−A) for a predetermined time, and the average value thereof iscomputed, and the reference value CER which is the average value of thefirst auxiliary signal ag is set in Step S51 in the normal state, andnext, when the optical pick-up unit is moved at a high speed, theoptical pick-up unit is moved in the radial direction of the track forthereby detecting the second auxiliary signal CED in Step S52, and thereference value CER and the detected second auxiliary signal CED arecompared. As a result of the comparison, if the reference value CED andthe second auxiliary signal CED are the same, Step S52 is repeatedlyperformed, and if the reference value CER and the second auxiliarysignal CED are not the same, the next step is performed in Step S53. Ifthe reference value CER and the second auxiliary signal CED are not thesame, and the tracking error signal TE is generated, a kick pulse signalor brake pulse signal are outputted for thereby implementing a trackingcompensation control in Step S54. At the same time, the differencesbetween the reference value CER and the detected second auxiliary signalCED are accumulated for thereby generating a slope value SD whichindicates the variation of the tracking error signal TE in Step S55.Next, the tracking error slope SD is compared with the reference slopevalue SR which was previously calculated in Step S56. As a result of thecomparison, if the tracking error slope value SD is not greater than thereference slope SR, it is judged that the objective lens is greatlymoved. The steps S54 and S56 are repeatedly performed until the trackingerror slope SD becomes less than the reference slope SR. If the trackingerror slope SD is less than the reference slop SR, the kick pulse orbrake signal are applied in a reverse sequence for thereby performing areverse compensation control in Step S57.

FIG. 14A illustrates the output signal waveforms of the photodetectorfor detecting the position of the objective lens when the opticalpick-up unit of the optical disc reproducer moves in the radialdirection of the tracks of the optical disc in the stable state, whichis the same as in FIG. 12A.

FIG. 14B illustrates the output signal waveforms of the positiondetection unit for detecting the position of the objective lens in whicheach level is respectively indicated by LD1, LD11, LD2, LD21, LD3, LD31,LD4 and LD41 corresponding to their respective positions at the timest1, t2, t3 and t4 in the reference signal CER and the detected signalCED, respectively. At this time, if the slope of the output signal ofthe photodetector is larger than a reference slope in the positive andnegative directions, the tracking of the objective lens is controlled byapplying the back electromotive force.

As shown in FIG. 14B, at the positions t1, t2, t3 and t4, the vibrationlevels corresponding to each position are indicated as LD1, LD2, LD3 andLD4. At this time, the level of the vibration represented in the outputsignal of the photodetector is compared with a previously set vibrationlevel for thereby applying a back electromotive force and controllingthe tracking of the objective lens. The above-described method will beexplained in more detail with reference to FIG. 15.

FIG. 15 is a flow chart illustrating the steps for implementing a trackaccess method which is capable of controlling the tracking using avibration level of the objective lens with respect to the elapsed timeaccording to a fifth embodiment of the present invention. As showntherein, in Step S61 when the optical pick-up unit runs at a normalspeed, the first auxiliary signals ag(C−A) are detected as the opticalpick-up unit moves in the track direction for a predetermined time, andthen the average value thereof is computed, and the thusly computedvalue is set as a reference value CER of the first auxiliary signal inthe normal running state (stable state), and next, when the opticalpick-up unit is moved at a high speed, as the optical pick-up unit ismoved in the radial direction of the track, the second auxiliary signalCED is detected in Step S62. The reference value CER and the value ofthe second auxiliary signal CED are compared. As a result of thecomparison, if the reference value CER and the value of the secondauxiliary signal CED are the same, Step S62 is continuously repeated. Ifthey are not the same, the next step is performed in Step S63. If thereference value CER and the value of the second auxiliary signal CED arenot the same, namely, when the tracking error signal occurs, the kickpulse signal or the brake pulse signal are outputted for therebyimplementing a tracking compensation control in Step S64. At the sametime, the difference (CER−CED) between the reference value CER and thevalue of the second auxiliary signal CED is obtained and a vibrationlevel value LD is generated by the difference in Step S65, and next, inStep S66 the vibration level value LD is compared with a reference levelLR which was previously determined. If the vibration level LD is notgreater than the reference level LR, Steps S64 and S65 are repeatedlyperformed. If the vibration level LD is less than or equal to thereference level LR, it is judged that the vibration of the objectivelens is small, so that in Step S67 the kick pulse signal or the brakepulse signal are tracking-controlled using a shorter duration outputsignal than applied in Step S64, so that the pick-up unit is positionedat the destination track in which an information of the optical disc isrecorded.

However, in the track access method which is capable of implementing atracking control using the time-based variation(slope) of the objectivelens position, if the tracking control is implemented so that the errorsignal at SD2 and SD4 is zero at the zero crossing point, the controlsignal applied based on the inertial force is too much increased, sothat the error signal is generated not to correspond to the position atwhich the error signal becomes zero, whereby the distortion is too muchincreased.

In the track access method which is capable of implementing the trackingcontrol using a vibration variation level of the objective lens positionwith respect to the elapsed time according to a fifth embodiment of thepresent invention, as illustrated in the drawings, LD1 and LD3 aredisplaced farther from the zero vibration level compared to LD2 and LD4,and LD1 and LD3 thus indicate the points at which the vibration is moregreatly generated and thereby more compensation control need beperformed, so that at LD2 and LD4, the tracking control signal isprocessed based on the decreased vibration level. If the trackingcontrol is performed based on the magnitude of the back electromotiveforce so that the error signal becomes zero at the points LD2 and LD4,since the control signal applied based on the inertial force isincreased, a level greatly different from the zero vibration level isobtained, so that the distortion is increased.

Another embodiment of the present invention which is provided forovercoming the problems encountered in the third and fourth embodimentsof the present invention will be explained with reference to FIG. 16.

FIG. 16 is a flow chart illustrating steps for implementing a trackaccess method for an optical disc reproducerin the fine search modeaccording to a sixth embodiment of the present invention. After thefirst tracking control operation in which the optical pick-up unit ismoved at a high speed is performed, when the reproducer is controlled sothat the second tracking control operation is implemented for accuratelymoving the optical pick-up unit, tracking error occurs due to theinertia force. The method for preventing the above-mentioned problemthat the tracking error occurs and implementing a fast fine search willbe explained.

When the mode is switched to the second tracking control operation mode,namely, the fine search mode in Step S71, when the optical pick-up unitis moved at a high speed, and then is stopped, the first auxiliarysignal CED, which has a magnitude and polarity which differ inaccordance with the amount of the beam detected by the segmentedphotodetector, is outputted and then its value is stored in Step S72. Itis judged that the objective lens is at the destination track when thevalue of CED is equal to zero in Step S73, and then the tracking controlis implemented in Step S75 based on the number of the jump tracks perthe moving track as a reference track number in Step S78, If the firstauxiliary signal CED is judged larger than zero in Step S74, it isjudged that the objective lens position is offset inwardly of thedestination track of the position of the optical disc in Step S76, andthe tracking control is implemented based on the number of the trackshigher than the jump track number per the moving track by N-tracks inStep S79. If the second auxiliary signal CED is judged less than zero inStep S74, it is judged that the objective lens position is offsetoutwardly of the destination track position of the optical disc in StepS77, so that the tracking control is implemented with respect to thejump track number per the moving track based on the number of trackswhich is less than the number of the reference tracks by N-tracks inStep S80, so that it is possible to implement a fast and fine trackingcontrol in the fine search mode.

In the tracking/controlled method according to the sixth embodiment ofthe present invention, when performing the tracking control of theDPD(Differential Phase Detection), and the optical pick-unit is movedacross the tracks, the vibration level signal having a resonantfrequency range of the tracking actuator is detected together with thereproducing signal for thereby detecting and compensating the objectivelens vibration level of a low frequency.

FIG. 17A illustrates a speed graph when the optical pick-up unit ismoved in the radial direction of the tracks. As shown therein, there areprovided an acceleration interval t1 over which the optical pick-up unitwhich was initially in the stopped state is moved, a constant speedinterval t2 over which the optical pick-up unit is moved at apredetermined speed, and a deceleration interval t4 over which theoptical pick-up unit which is moved at a constant speed is slowed andstopped.

FIG. 17B illustrates an acceleration graph when the optical pick-up unitis moved in the radial direction of the tracks. As shown therein, thereare provided an acceleration interval during which a force is applied tothe objective lens or actuator of the optical pick-up unit, and aconstant speed interval during which the force is not applied to theobjective lens or the actuator. At this time, after the force is appliedto the objective lens or the actuator, a vibration occurs in theactuator due to the inertial force of the optical pick-up unit. Inparticular, the tracking servo or the focusing servo may be lost due tothe thusly generated vibration for thereby delaying the data search.

FIG. 18 is a view illustrating the computation of the characteristicvalue K of the actuator. The microcontroller receives a traverse signalTS from the traverse counter when the optical pick-up unit is movedacross the tracks, for thereby obtaining the degree “r” of thedistortion of the actuator. At this time, the degree “r” of thedistortion is in proportion to the force applied to control theactuator. Therefore, since there is a proportional relationship betweenthe force and the degree of the distortion, a proportional constant “K”is obtained, which represents a characteristic of the actuator.Therefore, the relation F1=K*r is obtained between the force F1 appliedto the actuator and the degree of the distortion of the actuator. Here,since the proportional constant K is determined based on the mechanicalcharacteristics of the actuator and the force applied to the actuator,the proportional constant K is the characteristic value of the actuator.Therefore, when the characteristic value of the actuator is known, thedriving operation is controlled based on the degree of the distortion ofthe actuator, so that it is possible to implement a parallel state ofthe actuator. The control method of the same will be explained withreference to the accompanying drawings.

FIG. 19 is a flow chart illustrating steps for implementing the trackingcontrol method using the degree “r” of the distortion of the actuator.As shown therein, the method includes a step for obtaining thecharacteristic value of the actuator, and a step for detecting thedegree of the distortion of the actuator in accordance with a searchrequest, in which step the optical pick-up unit rapidly accesses thedestination track position of the optical disc. Each step will beexplained in more detail.

When the optical pick-up unit is moved in Step S91, the microcontrollerdetects a pulse generation signal PG generated based on the rotation ofthe sled motor, and detects the moving speed vo of the optical pick-upunit which moves in the radial direction of the tracks based on thetraverse signal TS in Step 92 for thereby computing the acceleration a0of the optical pick-up unit in Step S93 based on the moving speed v0. Atthis time, since the acceleration a0 is generated when a force isapplied to the optical pick-up unit, it is possible to calculate theforce F0 applied to the optical pick-up unit based on Newton's law ofmotion(F=m*a) based on the acceleration a0 and the mass mo of theoptical pick-up unit. In addition, the force F1 applied to the actuatormay be deduced based on the force F0 applied to the optical pick-upunit. Namely, F1 is obtained by multiplying the net mass m1 of theactuator(which is obtained by subtracting the mass m1 except for theactuator from the mass m0 of the entire portions of the optical pick-upunit) by the acceleration a0 of the optical pick-up unit. Next, thedegree “r” of the distortion of the actuator is obtained based on thetraverse signal TS in Step S94. Therefore, the microcontroller computesthe characteristic value K of the actuator based on the force F1corresponding to the acceleration a0 and the degree “r” of thedistortion of the actuator in Step S95.

The microcontroller stores the characteristic force K of the actuatorand in Step S96 judges whether there is a search request for searchingthe information recorded on the optical disc. If there is a searchrequest, the optical pick-up unit is moved in the radial direction ofthe tracks in Step S97. At this time, the microcontroller detects thecurrent moving speed of the optical pick-up unit in Step S98, and thecurrent acceleration is computed in Step S99 based on the current movingspeed. Since the current acceleration is directly related to the forceby which the optical pick-up unit is moved, the degree “r” of thedistortion of the actuator is computed in Step S100 based on the forceand the characteristic value K of the actuator. The actuator iscontrolled in Step S101 based on the back electromotive force which iscapable of controlling the actuator in accordance with the degree of thedistortion, and the optical pick-up unit accesses the destination trackat a high speed by off-setting compensating the distortion of theactuator.

In the above-described embodiment of the present invention, whentraversing the optical pick-up unit at a high speed in the optical discreproducer, the above-described access operation may be implementedunder the control of the microcontroller without an additionalapparatus.

In the track access apparatus and method according to the presentinvention, the slope of the detection signal from the position detectionunit of the objective lens position which is capable of detecting thevibration of the objective lens, the level variation degree, and thedegree of the distortion of the actuator are detected, and then the backelectromotive force corresponding to each physical amount is generated,so that the back electromotive force is applied for compensating theforce applied to the actuator. In addition, the vibration of theobjective lens is prevented in maximum, so that the optical pick-up unitrapidly access the destination track of the optical disc for therebysearching a desired data.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas recited in the accompanying claims.

1. An optical disc track access apparatus, comprising: a detection unitdetecting an auxiliary signal from a reflected main beam from an opticaldisc while an optical pick-up unit is crossing the tracks of the opticaldisc, wherein the auxiliary signal represents an amount of off-centerwith respect to an objective lens of the optical pickup unit and isdetermined without the use of a tracking error signal; and a controlunit controlling a driving operation of a tracking actuator inaccordance with the detected auxiliary signal.
 2. The apparatus of claim1, wherein said detection unit includes an error signal generation unitgenerating an off-center error signal in accordance with the detectedauxiliary signal, and wherein said control unit includes: a backelectromotive force generation unit generating a back electromotiveforce corresponding to the off-center error signal, and a drivingcontrol unit controlling the driving operation of the tracking actuatorin accordance with the generated back electromotive force.
 3. Theapparatus of claim 2, wherein said back electromotive force generationunit generates a back electromotive force in a direction for off-settingthe error signal.
 4. An optical disc track access apparatus for anoptical disc reproducer, comprising: a first tracking control means foroutputting a first tracking control signal based on a first auxiliarysignal generated in accordance with the track direction movement of anoptical pick-up unit of the optical disc reproducer; a second trackingcontrol means for outputting a second tracking control signal inaccordance with a second auxiliary signal of the frequency less than thefrequency of the first auxiliary signal generated based on the movementof the optical pick-up unit in a radial direction of the tracks of theoptical disc being reproduced; and a switching means for selecting oneamong the first and second tracking control signals and outputting thethusly selected signal in accordance with a track search request.
 5. Theapparatus of claim 4, wherein said second tracking control meansincludes: an auxiliary signal detection means for detecting the secondauxiliary signal of the resonant frequency range corresponding to avibration of a tracking actuator of the optical disc reproducer based onthe detection amount of a plurality of beam spot regions reflected fromthe optical disc; and an error signal generation means for generating atracking error signal in accordance with the second auxiliary signal. 6.The apparatus of claim 4, wherein said tracking control means generatesa back electromotive force corresponding to the first and secondauxiliary signals, and outputs the generated back electromotive force asa driving signal of the tracking actuator.
 7. An optical disc trackaccess apparatus, comprising: an auxiliary signal detection unitdetecting an auxiliary signal when a track search of an optical disc isperformed, wherein the detection unit detects the auxiliary signal froma main beam reflected from the optical disc, which represents an amountof off-center with respect to an objective lens and is determinedwithout the use of a tracking error signal; an error signal generationunit generating an off-center error signal in accordance with theauxiliary signal; and a driving control unit controlling a drivingsignal of a tracking actuator, wherein the driving signal generates aback electromotive force corresponding to the off-center error signal.8. An optical disc track access method, comprising the steps of:receiving a main beam reflected from an optical disc; detecting anauxiliary signal from the main beam while an optical pick-up unit iscrossing the tracks of the optical disc when a track search isperformed, wherein the auxiliary signal represents an amount ofoff-center with respect to an objective lens of the optical pickup unitand is determined without the use of a tracking error signal; andcontrolling a driving operation of a tracking actuator in accordancewith the auxiliary signal.
 9. The method of claim 8, wherein saidcontrolling step includes: generating a back electromotive forcecorresponding to the auxiliary signal; and controlling a driving signalof the tracking actuator in accordance with the generated backelectromotive force.